1. Field of the Invention
The invention relates to a method for shaping or treating a ceramic body including plural heating steps. More particularly the invention relates to a method for forming a polycrystalline aluminum oxynitride product. Most particularly, the invention relates to the formation of an aluminum oxynitride powder and a method for making the powder which provides better powder density, transparency, uniformity, reduced processing costs and equipment damage.
2. Discussion of the Related Art
The invention relates to the technology of making very dense ceramic materials. Certain ceramic materials have been used as ballistic armor. Any material used in this capacity must have certain physical properties in order to defeat a small arms projectile. Such a material must have a very hard face surface and be very dense. Polycrystalline aluminum oxynitride can be made to have these essential physical properties when made according to methods, which achieve this objective.
Polycrystalline aluminum oxynitride (AlON) are important ceramic materials used in making ballistic armor, bearings, and light transmitting envelopes. These materials can be used to replace current armors because of their increased mechanical and thermal shock resistance (see Martin et al., Synthesis and Hot Pressing of Transparent Aluminum Oxynitride, Window and Dome Technologies and Materials, SPIE Vol 1112, 1989). However, AlONs are limited from wide use because of manufacturing difficulties, particularly in making armor plate of the size and shape required and in the high manufacturing cost associated with large ceramic plate which must be free of flaws.
U.S. Pat. No. 4,241,000 to MCCauley et al. incorporated herein by reference, teach single phase (spinel) polycrystalline, cubic aluminum oxynitrides having isotropic optical, thermal, and electrical properties, which are physically and chemically stable at temperatures up to 1100° C. The method includes isostatic pressing Al2O3 and AlN, at 25,000 psi, and heat treating the compacted mixture at 1200° C. for 24 hours followed by a sintering step at temperatures above 2000° C.
U.S. Pat. No. 4,481,300 to Hartnett et al. and U.S. Pat. No. 4,686,070 to Maguire et al disclose polycrystalline doped cubic aluminum oxynitride materials and methods of forming AlON materials utilizing isostatic pressing.
While the above processes appear to provide AlON plates of up to 98% density, they have been inconsistent in providing the quality required for ballistic armors and the like. Additionally, these processes have been limited by the cost of manufacture and variation in product quality. Note that isostatic pressing involves placing a dry or semidry granulated ceramics mixture in a polymer mold and then uniformly pressing the mold while immersed in a high pressure oil or water cylinder (see Jones et al., “Ceramics, The Iowa State University Press, AMGS, 1972, pp. 52–61). The severe conditions associated with hydraulic systems and contamination of the dies is a major reason for the high cost of manufacture as well as for a notable product rejection rate.
U.S. Pat. No. 4,720,362 to Gentilman et al. disclose hot pressed cubic aluminum oxynitrides which provide transparent ceramic materials suitable for missile domes. Here, the powders are ball-milled with methanol, dried and calcined in a nitrogen atmosphere. The process forms aluminum oxynitride which is then ball milled with methanol to provide a single phase aluminum oxynitride powder. Boron and yttrium oxide may also be added as doping agents.
Hot pressing is a well known process for making ceramic powders having increased densities which are formed at relatively lower temperatures and shorter cycle times (see Martin et al, above). However, hot pressing is restrictive because unidirectional or bidirectional external pressures are required during processing. This leads to increased equipment costs particularly in tool wear and operation due to the high temperature and pressure conditions under which they are utilized (see Engineered Materials Handbook, Vol. 4, Ceramics and Glasses, pp 186–192).
The present invention overcomes the disadvantages of the prior methods of producing polycrystalline aluminum oxynitride (AlON) powders by providing a process for making AlONs that utilizes the benefits of hot pressing including short furnace cycle times, high product density and little green body preparation, without the shortcomings: tool wear and high operation costs.